Golf ball

ABSTRACT

The present invention, by forming the outermost layer of a golf ball cover from a specific resin composition that includes a thermoplastic resin and a color masterbatch having mutually similar specific gravities, provides a golf ball which is imparted with an excellent durability of markings thereon, a uniform and evenly colored appearance, and an excellent manufacturability.

BACKGROUND OF THE INVENTION

The present invention relates to a golf ball which has an excellent durability of markings thereon, a uniform and evenly colored appearance, and an excellent manufacturability.

Lately, not all golf balls have been white; balls in a variety of colors are being manufactured and marketed in response to the preferences of golfers. Such golf balls are generally colored by including a colorant such as a pigment or dye in the material used to form the outermost layer that serves as the ball cover. The cover is typically formed by feeding a base resin and various additives, colorant and the like in specific compounding ratios into an injection molding machine, thoroughly mixing these ingredients together within the molding machine, and injecting the resulting mixture into a given mold. However, if the resin, the various additives and the colorant have different specific gravities, they do not readily mix uniformly at this time within the injection molding machine, as a result of which color unevenness sometimes arises on the molded cover. The solution taken in such cases is to first prepare a masterbatch by mixing the colorant into the base resin. However, further improvement is required to better stabilize the quality.

Along with stabilizing the quality, it is also important to lower production costs for the ball. Accordingly, there exists a desire to find the production conditions that minimize trouble during ball manufacture and thus enable stable production to be carried out. In addition, the manufactured ball must also have an excellent durability of markings thereon. Hence, it is desired that markings on the ball not rub off during play and that the ball maintain a good appearance.

Prior art relating to this invention has been disclosed in, for example, JP-A 2012-105725, JP-A 2012-081111 and JP-A 2012-081110.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a golf ball which has an excellent durability of markings thereon, a uniform and evenly colored appearance, and an excellent manufacturability.

As a result of intensive investigations, the inventors have discovered that by optimizing the specific gravities of the base resin and the masterbatch included in the material used to form the outermost layer of a golf ball cover, and by also optimizing the amount of fatty acid metal salt included within this material, a golf ball having an excellent durability of markings thereon, a uniform and evenly colored appearance and an excellent manufacturability can be obtained.

Accordingly, the invention provides the following golf ball.

[1] A golf ball comprising a core and a cover of at least one layer encasing the core, wherein an outermost layer of the cover is formed of a resin composition comprising:

(A) a thermoplastic resin, and

(B) a color masterbatch,

which color masterbatch (B) includes

-   -   (b-1) a thermoplastic resin,     -   (b-2) a colorant,     -   (b-3) titanium oxide,     -   (b-4) a fatty acid metal salt, and     -   (b-5) a dispersant,         component (b-4) being included in an amount of from 0.3 to 1.4         parts by weight per 100 parts by weight of component (A) and         component (b-1) combined, the ratio (B)/(A) of the specific         gravity of component (A) to the specific gravity of         component (B) being from 0.8 to 1.2, and component (A) and         component (b-1) having weight-average molecular weights (Mw) of         at least 100,000.         [2] The golf ball of [1], wherein the thermoplastic resin (A) is         one or more selected from the group consisting of:     -   (a-1) an olefin-unsaturated carboxylic acid copolymer,     -   (a-2) a metal ion neutralization product of an         olefin-unsaturated carboxylic acid copolymer,     -   (a-3) an olefin-unsaturated carboxylic acid-unsaturated         carboxylic acid ester terpolymer, and     -   (a-4) a metal ion neutralization product of an         olefin-unsaturated carboxylic acid-unsaturated carboxylic acid         ester terpolymer.         [3] The golf ball of [2], wherein at least 30 wt % of the         unsaturated carboxylic acid included in (a-2) the metal ion         neutralization product of an olefin-unsaturated carboxylic acid         copolymer and/or (a-4) the metal ion neutralization product of         an olefin-unsaturated carboxylic acid-unsaturated carboxylic         acid ester terpolymer is neutralized with metal ions.         [4] The golf ball of [2], wherein (a-2) the metal ion         neutralization product of an olefin-unsaturated carboxylic acid         copolymer and/or (a-4) the metal ion neutralization product of         an olefin-unsaturated carboxylic acid-unsaturated carboxylic         acid ester terpolymer include as the metal ions one or more         selected from the group consisting of Li⁺, Na⁺, Mg⁺⁺, Ca⁺⁺ and         Zn⁺⁺.         [5] The golf ball of [2], wherein component (A) further includes         (a-5) one or more selected from the group consisting of         polyethylene, thermoplastic polyurethane and rubbery elastomer.         [6] The golf ball of [1], wherein the thermoplastic resin         serving as component (b-1) is one or more selected from the         group consisting of:     -   (b-1-1) an olefin-unsaturated carboxylic acid copolymer, and     -   (b-1-2) an olefin-unsaturated carboxylic acid-unsaturated         carboxylic acid ester terpolymer.         [7] The golf ball of [1], wherein the thermoplastic resin         serving as component (b-1) has a melt flow rate (MFR) of from 5         to 60 g/10 min.         [8] The golf ball of [1], wherein the colorant (b-2) is one or         more selected from the group consisting of red, blue, yellow,         orange, green, violet and black colorants.         [9] The golf ball of [1], wherein the fatty acid metal salt         (b-4) is a metal salt of a fatty acid having from 16 to 30         carbons.         [10] The golf ball of [1], wherein the fatty acid component of         the fatty acid metal salt (b-4) is one or more selected from the         group consisting of stearic acid, oleic acid, behenic acid and         montanic acid.         [11] The golf ball of [1], wherein the metal ion of the fatty         acid metal salt (b-4) is one or more selected from the group         consisting of Li⁺, Na⁺, Mg⁺⁺, Ca⁺⁺ and Zn⁺⁺.         [12] The golf ball of [1], wherein component (B) is included in         an amount of from 3 to 25 parts by weight per 100 parts by         weight of component (A).

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic cross-sectional diagram showing an example of the construction of a golf ball according to the invention.

FIG. 2 is a top view showing the markings that were placed on the golf balls fabricated in the examples of the invention and the comparative examples.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below.

The golf ball of the invention has a cover with an outermost layer that is formed using a specific resin composition which includes a thermoplastic resin and a color masterbatch having mutually similar specific gravities. The ball construction, so long as it includes a core and a cover of at least one layer, may be suitably set within a range that satisfies the objects of the invention. FIG. 1 shows an exemplary construction of the golf ball of the invention. The golf ball G shown in FIG. 1 is a two-piece solid golf ball having a core 1 and a cover 2 that encases the core. The cover 2 has a surface on which, typically, a plurality of dimples D are formed. In the diagram, the core 1 and the cover 2 are each formed as single layers, although either or both may be composed of a plurality of layers.

The golf ball of the invention is described in detail below while referring to FIG. 1.

First, the material used to form the core 1 is described. The core 1 is obtained by vulcanizing a rubber composition made up primarily of a rubber material. Specifically, use may be made of a rubber composition containing, for example, a base rubber, a crosslinking initiator and a co-crosslinking agent.

The base rubber of the rubber composition is not subject to any particular limitation, although the use of polybutadiene is preferred. Preferred use may be made of cis-1,4 polybutadiene having a cis structure content of at least 40%. If so desired, in the base rubber, other rubbers such as natural rubber, polyisoprene rubber or styrene-butadiene rubber may be suitably compounded with the above polybutadiene.

Preferred use may be made of an organic peroxide as the crosslinking initiator in the present invention. Illustrative examples of suitable organic peroxides include 1,1-di(t-butylperoxy)cyclohexane, 1,1-bis-t-butylperoxy-3,3,5-trimethylcyclohexane, dicumyl peroxide, di(t-butylperoxy)-meta-diisopropylbenzene and 2,5-dimethyl-2,5-di-t-butylperoxyhexane. A commercial product may be used as the organic peroxide. Examples of such products include Perhexa C-40, Perhexa 3M-40 and Percumyl D (available from NOF Corporation) and Trigonox 29-40 (available from Kayaku Akzo Corporation). These crosslinking initiators are compounded in an amount which, although not subject to any particular limitation, is preferably at least 0.1 part by weight, and more preferably at least 0.3 part by weight, per 100 parts by weight of the base rubber. The upper limit is preferably not more than 5 parts by weight, and more preferably not more than 2 parts by weight.

The co-crosslinking agent used in the invention may be, for example, a metal salt of an unsaturated fatty acid such as methacrylic acid or acrylic acid (e.g., zinc salts, magnesium salts, calcium salts), or an ester compound such as trimethylolpropane trimethacrylate. To obtain a high rebound in particular, preferred use may be made of zinc acrylate. The amount of such co-crosslinking agents included per 100 parts by weight of the base rubber, although not subject to any particular limitation, may be set to at least 10 parts by weight, and preferably at least 15 parts by weight, but not more than 50 parts by weight, and preferably not more than 40 parts by weight.

In addition, various additives may be optionally included in the above composition, examples of such additives being sulfur, antioxidant, zinc oxide, barium sulfate, the zinc salt of pentachlorothiophenol, and zinc stearate. No particular limitation is imposed on the amounts in which these additives are included.

The core diameter, although not particularly limited, may be set to preferably at least 32.0 mm, and more preferably at least 33.0 mm. The upper limit is preferably not more than 42.0 mm, and more preferably not more than 41.0 mm. The core coloration also is not subject to any particular limitation.

Next, the material used to form the cover 2 is described.

In this invention, the cover-forming material is a resin composition which includes:

(A) a thermoplastic resin, and

(B) a color masterbatch.

The thermoplastic resin serving as component (A) is not subject to any particular limitation, although preferred use can be made of one or more selected from the group consisting of:

-   -   (a-1) an olefin-unsaturated carboxylic acid copolymer,     -   (a-2) a metal ion neutralization product of an         olefin-unsaturated carboxylic acid copolymer,     -   (a-3) an olefin-unsaturated carboxylic acid-unsaturated         carboxylic acid ester terpolymer, and     -   (a-4) a metal ion neutralization product of an         olefin-unsaturated carboxylic acid-unsaturated carboxylic acid         ester terpolymer.         In this invention, (a-5) one or more selected from the group         consisting of polyethylene, thermoplastic polyurethane and         rubbery elastomer may be additionally included.

In the olefin unsaturated carboxylic acid copolymer (component a-1) of above component (A), the number of carbons on the olefin is typically at least 2 and not more than 8, and is preferably not more than 6. Specific examples include ethylene, propylene, butene, pentene, hexene, heptene and octene. Ethylene is especially preferred. Examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid (anhydride) and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

Component (a-2) is obtained by neutralizing the unsaturated carboxylic acid included in above component (a-1) with a metal ion. The metal ion which neutralizes the unsaturated carboxylic acid is preferably one or more metal ion selected from the group consisting of Li⁺, Na⁺, K⁺, Mg⁺⁺, Ca⁺⁺, Zn⁺⁺, Cu⁺⁺, Ba⁺⁺, Pb⁺⁺ and Al⁺⁺⁺. Neutralization with one or more metal ion selected from the group consisting of Li⁺, Na⁺, Mg⁺⁺, Ca⁺⁺ and Zn⁺⁺ is especially preferred.

Although not subject to any particular limitation, it is recommended that preferably at least 30 wt %, more preferably at least 40 wt %, and even more preferably at least 50 wt %, of the unsaturated carboxylic acid included in the copolymer of component (a-2) be neutralized with metal ions.

In the olefin unsaturated carboxylic acid-unsaturated carboxylic acid ester terpolymer serving as component (a-3), the olefin and the unsaturated carboxylic acid may be the same as in component (a-1). As the unsaturated carboxylic acid ester, preferred use can be made of, for example, lower alkyl esters of the above-described unsaturated carboxylic acids. Illustrative examples include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. The use of butyl acrylate (n-butyl acrylate, isobutyl acrylate) is especially preferred.

Component (a-4) is obtained by neutralizing the unsaturated carboxylic acid included in above component (a-3) with a metal ion. The metal ion which neutralizes the unsaturated carboxylic acid is exemplified by the same ions as in component (a-2). Preferred examples include metal ions selected from the group consisting of Li⁺, Na⁺, K⁺, Mg⁺⁺, Zn⁺⁺, Cu⁺⁺, Ba⁺⁺, Pb⁺⁺ and Al⁺⁺⁺. Neutralization with a metal ion selected from the group consisting of Li⁺, Na⁺, Mg⁺⁺, Ca⁺⁺ and Zn⁺⁺ is especially preferred.

Although not subject to any particular limitation, as in component (a-2), it is recommended that preferably at least 30 wt %, more preferably at least 40 wt %, and even more preferably at least 50 wt %, of the unsaturated carboxylic acid included in the terpolymer of component (a-4) be neutralized with metal ions.

Component (a-5) is one or more component selected from the group consisting of polyethylene, thermoplastic polyurethane and rubbery elastomer. Known substances may be used as any of these. Although not subject to any particular limitation, especially preferred examples of rubbery elastomers include styrene-butadiene copolymers (SBR), styrene-butadiene-styrene copolymers (SBS), hydrogenated styrene-butadiene copolymers (H-SBR), styrene-ethylene/butene-styrene copolymers (SEBS), hydrogenated styrene-isoprene-styrene copolymers (SEPS), styrene-isoprene/butadiene-styrene copolymers (SIBS), hydrogenated styrene-isoprene/butadiene-styrene copolymers (SEEPS), styrene-isoprene-styrene copolymers (SIS), hydrogenated styrene-ethylene/butylene-olefin crystalline block copolymers (SEBC) and hydrogenated olefin crystalline block-ethylene/butylene-olefin crystalline block copolymers (CEBC).

The weight-average molecular weight (Mw) of above component (A) may be set to at least 100,000, and preferably at least 110,000. The upper limit may be set to preferably not more than 200,000, and more preferably not more than 190,000. If the weight-average molecular weight (Mw) is too small, the flow properties of the material may be too high, which often leads to a loss of homogeneity in the material at the time of injection molding. Moreover, obtaining a resilience and durability to impact sufficient for a golf ball is difficult. On the other hand, if the weight-average molecular weight (Mw) is too large, the flow properties may become too low, which may make it impossible to obtain a good moldability.

The above copolymer has a weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio which, although not particularly limited, may be set to preferably at least 2.0, and more preferably at least 2.5. The upper limit may be set to preferably not more than 10.0, and more preferably not more than 9.5.

Here, the weight-average molecular weight (Mw) and number-average molecular weight (Mn) are values calculated relative to polystyrene in gel permeation chromatography (GPC). A word of explanation is needed here concerning GPC molecular weight measurement. It is not possible to directly take GPC measurements for copolymers and terpolymers because these molecules are adsorbed to the GPC column owing to unsaturated carboxylic acid groups within the molecules. Instead, the unsaturated carboxylic acid groups are generally converted to esters, following which GPC measurement is carried out and the polystyrene-equivalent average molecular weights Mw and Mn are calculated.

A commercial product may be used as component (A). Illustrative examples include the following.

Illustrative examples of component (a-1) include ethylene-methacrylic acid copolymers such as Nucrel N1035, Nucrel N1525, Nucrel N1560, Nucrel N1050H, Nucrel N1110H and Nucrel N2050H (all available from DuPont-Mitsui Polychemicals Co., Ltd.), and ethylene-acrylic acid copolymers such as Primacor 59801 (Dow Chemical).

Illustrative examples of component (a-2) include Himilan 1601 (Na), Himilan 1555 (Na), Himilan 1557 (Zn), Himilan 1605 (Na), Himilan 1706 (Zn), Himilan 1707 (Na), Himilan AM7311 (Mg) and Himilan AM7329 (Zn) (all available from DuPont-Mitsui Polychemicals Co., Ltd.). Additional examples include Surlyn 8945 (Na), Surlyn 9945 (Zn), Surlyn 8150 (Na), Surlyn 9120 (Zn), Surlyn 9150 (Zn), Surlyn 6910 (Mg), Surlyn 6120 (Mg), Surlyn 7930 (Li) and Surlyn 7940 (Li) (all available from E.I. DuPont de Nemours & Co.); and Iotek 8000 (Na), Iotek 8030 (Na), Iotek 7010 (Zn) and Iotek 7030 (Zn) (all available from ExxonMobil Chemical).

Illustrative examples of component (a-3) include Nucrel AN4318 and Nucrel AN4319 (DuPont-Mitsui Polychemicals Co., Ltd.), Nucrel AE (E.I. DuPont de Nemours & Co.), and Primacor AT310 and Primacor AT320 (Dow Chemical).

Illustrative examples of component (a-4) include Himilan AM7327 (Zn), Himilan 1855 (Zn), Himilan 1856 (Na) and Himilan AM7331 (Na) (DuPont-Mitsui Polychemicals Co., Ltd.). Additional examples include terpolymeric ionomer resins such as Surlyn 6320 (Mg), Surlyn 8320 (Na) and Surlyn 9320 (Zn) (E. I. DuPont de Nemours & Co.), and Iotek 7510 (Zn) and Iotek 7520 (Zn) (ExxonMobil Chemical).

The symbols Na, Zn and Mg appearing in parentheses after the trade names indicate the type of neutralizing metal ion.

Illustrative examples of component (a-5) include the following commercial products.

First, exemplary polyethylenes include the high-pressure low-density polyethylenes UBE Polyethylene R300, UBE Polyethylene F120N and UBE Polyethylene J1019 (all available from Ube-Maruzen Polyethylene). Exemplary thermoplastic polyurethanes include Pandex T8180, Pandex T8195, Pandex T8290, Pandex T8295 and Pandex T8260 (all available from DIC Bayer Polymer, Ltd.), and Resamine 2593 and Resamine 2597 (Dainichiseika Color & Chemicals Mfg. Co., Ltd.). Exemplary rubbery elastomers include the hydrogenated polymers Dynaron 6100P and Dynaron 6200P (JSR Corporation).

The specific gravity of component (A), although not particularly limited, may be set to preferably at least 0.8, and more preferably at least 0.85. The upper limit may be set to preferably not more than 1.2, and more preferably not more than 1.15.

The melt flow rate (MFR) of component (A) is not subject to any particular limitation. However, from the standpoint of providing good flow properties and molding processability during injection molding, the MFR, as measured in accordance with JIS K-7210 at a test temperature of 190° C. and a test load of 21.18 N (2.16 kgf), is set to preferably at least 0.1 g/10 min, more preferably at least 0.5 g/10 min, and even more preferably at least 0.8 g/10 min. It is recommended that the upper limit be set to preferably not more than 30 g/10 min, more preferably not more than 25 g/10 min, and even more preferably not more than 20 g/10 min. In cases where the MFR of component (A) is too large, the flow properties of the material may become too high, which often leads to a loss in the homogeneity of the material during injection molding. Moreover, obtaining a resilience and durability to impact sufficient for a golf ball may become difficult. On the other hand, if the MFR of component (A) is too small, the flow properties may become too low, as a result of which a good moldability may not be attainable.

The color masterbatch serving as component (B) is obtained by mixing a colorant with a known resin and additives. In this invention, advantageous use may be made of a resin composition containing the following components (b-1) to (b-5):

-   -   (b-1) a thermoplastic resin,     -   (b-2) a colorant,     -   (b-3) titanium oxide,     -   (b-4) a fatty acid metal salt, and     -   (b-5) a dispersant.

A known resin may be used as the thermoplastic resin serving as component (b-1). Although not subject to any particular limitation, preferred use may be made of one or more selected from the group consisting of:

-   -   (b-1-1) an olefin-unsaturated carboxylic acid copolymer, and     -   (b-1-2) an olefin-unsaturated carboxylic acid-unsaturated         carboxylic acid ester terpolymer.

The olefin-unsaturated carboxylic acid copolymer serving as component (b-1-1) is exemplified by the same copolymers as component (a-1) above. In this case, the number of carbons on the olefin included in component (b-1-1) is typically at least 2 and not more than 8, and is preferably not more than 6. Specific examples of such olefins include ethylene, propylene, butene, pentene, hexene, heptene and octene. Ethylene is especially preferred. Examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid (anhydride) and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

The olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester terpolymer serving as component (b-1-2) is exemplified by the same copolymers as component (b-1-1) above. As the unsaturated carboxylic acid ester, preferred use can be made of, for example, lower alkyl esters of the above-mentioned unsaturated carboxylic acids. Illustrative examples include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. The use of butyl acrylate (n-butyl acrylate, isobutyl acrylate) is especially preferred.

The weight-average molecular weight (Mw) of above component (b-1) may be set to at least 100,000, and preferably at least 110,000. The upper limit may be set to preferably not more than 200,000, and more preferably not more than 190,000. If the weight-average molecular weight (Mw) is too small, the flow properties of the material may be too high, which often leads to a loss of homogeneity in the material at the time of injection molding and to color unevenness. Moreover, obtaining a resilience and durability to impact sufficient for a golf ball is difficult. On the other hand, if the weight-average molecular weight (Mw) is too large, the flow properties may become too low, making it impossible to obtain a good moldability, in addition to which the dispersibility of the colorant may worsen, which may give rise to color unevenness.

The above copolymer has a weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio which, although not particularly limited, is set to preferably at least 2.0, and more preferably at least 2.5. The upper limit is preferably set to not more than 10.0, and more preferably not more than 9.5.

A commercial product may be used as component (b-1). Illustrative examples include the following.

Examples of component (b-1-1) include ethylene-methacrylic acid copolymers such as Nucrel N1035, Nucrel N1525, Nucrel N1560, Nucrel N1050H, Nucrel N1110H and Nucrel N2050H (all available from DuPont-Mitsui Polychemicals Co., Ltd.), and ethylene-acrylic acid copolymers such as Primacor 59801 (Dow Chemical).

Examples of component (b-1-2) include Nucrel AN4318 and Nucrel AN4319 (both available from DuPont-Mitsui Polychemicals Co., Ltd.), Nucrel AE (E.I. DuPont de Nemours & Co.), and Primacor AT310 and Primacor AT320 (Dow Chemical).

The content of component (b-1) in component (B), although not particularly limited, is preferably set to at least 10 wt %, and is more preferably at least 15 wt %, and even more preferably at least 20 wt %. The upper limit is preferably set to not more than 99.9 wt %, and is more preferably not more than 99.7 wt %, and even more preferably not more than 99.5 wt %. If the content of component (b-1) is too small, it may not be possible to obtain pellets of a suitable shape during preparation of the color masterbatch, as a result of which charging of the color masterbatch into the molding machine during molding of the cover may be difficult. On the other hand, if the content of component (b-1) is too large, the proportion of component (b-1) relative to the contents of components (A) and (B) increases, which may make it difficult to obtain a resilience sufficient for a golf ball.

The melt flow rate (MFR) of component (b-1) is not subject to any particular limitation. However, from the standpoint of providing good flow properties and molding processability during injection molding, the MFR, as measured in accordance with JIS K-7210 at a test temperature of 190° C. and a test load of 21.18 N (2.16 kgf), may be set to preferably at least 5 g/10 min, more preferably at least 7 g/10 min, and even more preferably at least 10 g/10 min. It is recommended that the upper limit be set to preferably not more than 60 g/10 min, more preferably not more than 55 g/10 min, and even more preferably not more than 50 g/10 min. In cases where the MFR of component (b-1) is too large (the flow properties are too high), this often leads to a loss of homogeneity in the material during injection molding, and color unevenness. Moreover, obtaining a resilience and durability to impact sufficient for a golf ball may be difficult. In cases where the MFR of component (b-1) is too small (the flow properties are too low), it may not be possible to obtain a good moldability, in addition to which the dispersibility of the colorant may worsen and color unevenness may arise.

The colorant serving as component (b-2) is included for the purpose of adjusting the color of the ball. In the practice of the invention, a known pigment or dye may be used. From the standpoint of resistance to discoloration, it may be preferable to use a pigment. The colorant is suitably selected from the group consisting of red colorants, blue colorants, yellow colorants, orange colorants, green colorants, violet colorants and black colorants. Exemplary red colorants include those based on quinacridones, perylenes and anthraquinones; exemplary blue colorants include those based on phthalocyanines and ultramarine; exemplary yellow colorants include complex oxide colorants, polyazo colorants and heterocyclic colorants; exemplary violet colorants include ultramarine violet; and black colorants include carbon black. By including these, it is possible to express a color having a sense of elegance. These color pigments or dyes may be used singly, or two or more may be used together; the combinations or included amounts of the color pigments or dyes may be suitably selected according to the ball specifications and other considerations. Commercial products may be used as the color pigment or dye. Specific examples of such commercial products include those available under the trade names CHROMOFINE (Dainichi Seika Color & Chemicals Mfg. Co., Ltd.), TIPAQUE YELLOW (Ishihara Sangyo Kaisha, Ltd.) and TOKABLACK (Tokai Carbon Co., Ltd.). The content of component (b-2) in component (B), although not particularly limited, is preferably set to at least 0.1 wt %, more preferably at least 0.2 wt %, and even more preferably at least 0.3 wt %. The upper limit is preferably set to not more than 90 wt %, more preferably not more than 80 wt %, and even more preferably not more than 70 wt %. If the content of component (b-2) is too low, a sufficient coloration may not be obtained. On the other hand, if the content of component (b-2) is too high, it may not be possible to obtain pellets of a suitable shape, as a result of which charging of the color masterbatch into the molding machine during molding of the cover may be difficult. Moreover, obtaining a resilience and durability to impact sufficient for a golf ball may be difficult.

The titanium oxide used as component (b-3) may be a known titanium oxide. For example, preferred use may be made of the products commercially available under the trade name TIPAQUE (from Ishihara Sangyo Kaisha). The content of component (b-3) in component (B), although not particularly limited, is set to preferably at least 0.01 wt %, more preferably at least 0.5 wt %, and even more preferably at least 0.1 wt %. The upper limit is preferably set to not more than 30 wt %, and is more preferably not more than 20 wt %, and even more preferably not more than 10 wt %.

The fatty acid metal salt serving as component (b-4) is not particularly limited, although preferred use may be made of a metal salt of a fatty acid having from 16 to 30 carbons. The metal ion included in the fatty acid metal salt may be one or more selected from the group consisting of Li⁺, Na⁺, Mg⁺⁺, Ca⁺⁺ and Zn⁺⁺. Illustrative examples include the sodium salts, zinc salts, magnesium salts, calcium salts and lithium salts of stearic acid, oleic acid, behenic acid and montanic acid. In the practice of the invention, the use of magnesium stearate, calcium stearate, zinc stearate, magnesium behenate or magnesium montanate is especially preferred. The content of component (b-4) in component (B), although not particularly limited, may be set to preferably at least 1.8 wt %, more preferably at least 2.4 wt %, and even more preferably at least 3.0 wt %. The upper limit is preferably set to not more than 29.0 wt %, more preferably not more than 27.0 wt %, and even more preferably not more than 25.0 wt %.

The content of above component (b-4) in the overall cover-forming material must be set to at least 0.3 part by weight per 100 parts by weight of above component (A) and component (b-1) combined; it is recommended that this content by set to preferably at least 0.4 part by weight, and more preferably at least 0.5 part by weight. The upper limit must be set to not more than 1.4 parts by weight; it is recommended that this upper limit be set to not more than 1.3 parts by weight, and more preferably not more than 1.2 parts by weight. If the content of component (b-4) is too low, sufficient releasability from the mold may not be obtained. On the other hand, if the content of component (b-4) is too high, the durability of markings on the golf ball cover may decrease.

A known dispersant may be used as the dispersant serving as component (b-5). Although not subject to any particular limitation, preferred use can be made of, for example, a polyethylene wax or a cationic group-containing acrylic polymer. The content of above component (b-5) in component (B), although not particularly limited, is preferably set to at least 0.01 wt %, more preferably at least 0.5 wt %, and even more preferably at least 0.1 wt %. The upper limit is preferably set to not more than 10.0 wt %, more preferably not more than 9.0 wt %, and even more preferably not more than 8.0 wt %. If the content of component (b-5) is too low, the dispersibility of the colorant and titanium oxide in the masterbatch may worsen. On the other hand, if the content of component (b-5) is too high, it may not be possible to obtain pellets of a suitable shape during preparation of the color masterbatch, as a result of which charging of the color masterbatch into the molding machine during molding of the cover may be difficult. Moreover, obtaining a resilience and durability to impact sufficient for a golf ball may be difficult.

The specific gravity of component (B), although not particularly limited, may be set to preferably at least 0.8, and more preferably at least 0.85. The upper limit may be set to preferably not more than 1.2, and more preferably not more than 1.15.

The ratio (B)/(A) of the specific gravity of component (A) to the specific gravity of component (B) must be at least 0.8, and may be set to preferably at least 0.85. The upper limit must be not more than 1.2, and may be set to preferably not more than 1.15. When the specific gravity ratio falls outside of the above range, the material may not mix uniformly and color unevenness may arise in the cover after molding.

The amount of component (B) included in the cover-forming material, although not particularly limited, is preferably at least 3 parts by weight, more preferably at least 4 parts by weight, and even more preferably at least 5 parts by weight, per 100 parts by weight of component (A). The upper limit is preferably set to not more than 25 parts by weight, and is more preferably not more than 23 parts by weight, and even more preferably not more than 20 parts by weight. If the amount of component (B) included is too low, the material may not mix uniformly and color unevenness may arise in the cover after molding. On the other hand, if the amount of component (B) included is too high, the proportion of component (b-1) relative to the content of components (A) and (B) becomes high, which may make it difficult to obtain a resilience sufficient for a golf ball.

The color masterbatch serving as component (B) may be prepared by a known method. For example, the color masterbatch may be obtained by mixture using a mixing apparatus selected from among twin-screw/single-screw extruders (including kneader-extruders) equipped with a pressurizing kneader and a force feeder, tandem extruders, and twin-screw extruders. These extruders need not be of a special design; the use of an existing extruder will suffice.

In addition, various types of additives, such as ultraviolet absorbers, antioxidants, metal soaps and inorganic fillers other than those mentioned above, may be suitably included in this cover-forming material.

Preparation of the resin composition containing above components (A) and (B) may be carried out using a known mixing apparatus, such as a single-screw extruder or a twin-screw extruder. In this invention, the use of a twin-screw extruder is preferred. Alternatively, these extruders may be used in a tandem arrangement, such as single-screw extruder/twin-screw extruder or twin-screw extruder/twin-screw extruder. These extruders need not be of a special design; the use of an existing extruder will suffice. The method of molding the cover using the above resin composition is not subject to any particular limitation. For example, use may be made of an injection molding process or a compression molding process. In cases where an injection molding process is used, the process may be one in which a prefabricated core is placed at a predetermined position in an injection mold, following which the resin composition is introduced into the mold.

Alternatively, in cases where a compression-molding process is used, the process may be one in which a pair of half-cups are fashioned beforehand by an injection-molding process or the like from the above resin composition, following which the cups are placed over the core, and heat and pressure are applied in a mold. When molding is carried out under applied heat and pressure, the molding conditions employed may be a temperature of from 120 to 170° C. and a molding time of from 1 to 5 minutes.

The thickness per layer of the cover may be suitably set without particular limitation according to such considerations as the construction and materials of the golf ball to be produced, although the thickness is set to preferably at least 0.1 mm, and more preferably at least 0.2 mm. There is no particular upper limit, although the thickness per cover layer may be set to preferably not more than 4 mm, and more preferably not more than 3 mm.

In cases where a multi-piece solid golf ball having a cover of two or more layers formed over a core is to be produced, known resin materials may be used without particular limitation to form the layer or layers of the cover other than the outermost layer (i.e., intermediate layer or layers).

In the above golf ball, the shapes, number and arrangement of the dimples formed on the surface may be suitably set according to the ball specifications, and are not subject to any particular limitations. For example, the dimple shapes may be suitably selected from among not only circular shapes, but also non-circular polygonal shapes, dewdrop shapes and oval shapes. The diameter of the above dimples, although not particularly limited, is preferably set in the range of 0.5 to 6 mm. In addition, the dimple depth, although not particularly limited, is preferably set in the range of 0.05 to 0.4 mm.

No particular limitation is imposed on the surface coverage by the dimples on the surface of the ball. However, from the standpoint of the aerodynamic properties, the surface coverage is preferably set to at least 70%, more preferably at least 75%, and even more preferably at least 80%.

The ball has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) which, although not particularly limited, may be set to preferably at least 2.5 mm. The upper limit in the deflection may be set to preferably not more than 7 mm.

The golf ball of the invention can be made to conform to the Rules of Golf for competitive play. Specifically, the ball can be formed to a diameter which is not less than 42.67 mm and to a weight which is not more than 45.93 g.

As explained above, the present invention is able to provide a golf ball which has an excellent durability such that markings on the ball do not rub off during play, and which also has a uniform and evenly colored appearance. Moreover, because this golf ball has an excellent releasability from the mold and can be stably produced, the ball also has an outstanding manufacturability.

EXAMPLES

Working Examples of the invention and Comparative Examples are given below by way of illustration, and not by way of limitation.

Examples 1 to 10, Comparative Examples 1 to 3

Cores having a diameter of 38.55 mm were produced by preparing a rubber composition formulated as shown in Table 1, using a kneader or a roll mill to masticate the composition, then vulcanizing the composition at 155° C. for 17 minutes.

TABLE 1 A B Formulation Polybutadiene rubber 100 100 (parts by weight) Zinc oxide 4 4 Barium sulfate 20.8 21 Titanium oxide 2 Antioxidant 0.1 0.1 Zinc salt of pentachlorothiophenol 0.6 0.3 Zinc acrylate 25 23 Organic Peroxide 1 0.6 0.6 Organic Peroxide 2 0.6 0.6

Details on the ingredients shown in Table 1 are given below.

-   Polybutadiene rubber: Available as “BR01” from JSR Corporation -   Zinc oxide: Available from Sakai Chemical Co., Ltd. -   Barium sulfate: Available as “Precipitated Barium Sulfate 100” from     Sakai Chemical Co., Ltd. -   Titanium oxide: Available as “Tipaque R550” from Ishihara Sangyo     Kaisha, Ltd. -   Antioxidant: Available as “Nocrac NS-6” from Ouchi Shinko Chemical     Industry Co., Ltd. -   Zinc acrylate: Available from Nihon Jyoryu Kogyo Co., Ltd. -   Organic Peroxide 1: Dicumyl peroxide, available as “Percumyl D” from     NOF Corporation -   Organic Peroxide 2: 1,1-Bis(t-butylperoxy)cyclohexane diluted to 40%     with an inorganic filler, available as “Perhexa C-40” from NOF     Corporation; half-life at 155° C., about 50 seconds

Next, a color masterbatch or masterbatch formulated as shown in Table 2 was prepared.

TABLE 2 No. 1 No. 2 No. 3 Formulation Nucrel N1035 92.03 74.80 (parts by weight) Nucrel N1050H 31.22 Titanium oxide 0.3 0.6 54 Polyethylene wax 0.23 5.93 0.51 Yellow dye 0.61 Yellow pigment 12 Blue pigment 1.26 Magnesium stearate 6.83 6.67 13.01

Details on the ingredients shown in Table 2 are given below.

-   Nucrel N1035: An ethylene-methacrylic acid copolymer available from     DuPont-Mitsui -   Polychemicals Co., Ltd.; Mw, 168,000; MFR, 35 g/10 min -   Nucrel N1050: An ethylene-methacrylic acid copolymer available from     DuPont-Mitsui Polychemicals Co., Ltd.; Mw, 61,000; MFR, 500 g/10 min -   Titanium oxide: Available as “Tipaque R550” from Ishihara Sangyo     Kaisha, Ltd. -   Polyethylene wax: Available as “Sanwax 161P” from Sanyo Chemical     Industries, Ltd. -   Magnesium stearate: Available as “Magnesium Stearate G” from NOF     Corporation

A cover-forming material containing the color masterbatch or masterbatch shown in Table 2 and the ingredients shown in Tables 3 to 5 was then injection-molded over the core obtained as described above, thereby forming a cover having a thickness of 2.07 mm, and the markings shown in FIG. 2 were thermal transfer printed onto the surface thereof. Although not particularly described herein, numerous dimples were formed in identical patterns on the surfaces of the golf balls in each of the Working Examples of the invention and the Comparative Examples.

TABLE 3 Example 1 2 3 4 5 6 Core Material A A A A A A Diameter (mm) 38.55 38.55 38.55 38.55 38.55 38.55 Cover Thickness (mm) 2.07 2.07 2.07 2.07 2.07 2.07 Color yellow yellow yellow yellow yellow yellow Type of masterbatch dye dye dye dye dye dye colorant Formulation (A) Himilan 1601 42.5 42.5 42.5 42.5 42.5 42.5 (pbw) Himilan 1557 42.5 42.5 42.5 42.5 42.5 42.5 Himilan 1605 Himilan 1706 Nucrel AN4319 15 15 15 15 15 15 (B) Color 5 7.5 10 15 20 25 masterbatch No. 1 Color masterbatch No. 2 (B)′ Masterbatch No. 3 Magnesium stearate* 0.33 0.48 0.63 0.90 1.15 1.39 Weight-average (A) 125,980 125,980 125,980 125,980 125,980 125,980 molecular weight (b-1) 168,000 168,000 168,000 168,000 168,000 168,000 Specific (A) 0.95 0.95 0.95 0.95 0.95 0.95 gravity (B) and/or (B)′ 0.94 0.94 0.94 0.94 0.94 0.94 Specific gravity ratio ((B) and/or (B)′)/(A) 0.99 0.99 0.99 0.99 0.99 0.99 Evaluations Ball deflection (mm) 3.36 3.36 3.35 3.35 3.35 3.34 Releasability from mold good good good good good good Durability of markings good good good good good fair Color uniformity good good good good good good *The amount of magnesium stearate (component (b-4)) included in the masterbatch is given here as the content per 100 parts by weight of component (A) and component (b-1) combined.

TABLE 4 Example 7 8 9 10 Core Material B B B B Diameter (mm) 38.55 38.55 38.55 38.55 Cover Thickness (mm) 2.07 2.07 2.07 2.07 Color yellow yellow yellow yellow Type of masterbatch colorant dye dye dye dye Formulation (A) Himilan 1601 50 20 50 (pbw) Himilan 1557 60 80 50 Himilan 1605 40 Himilan 1706 50 Nucrel AN4319 (B) Color masterbatch No. 1 Color masterbatch No. 2 15 15 15 15 (B)′ Masterbatch No. 3 Magnesium stearate* 0.90 0.90 0.90 0.90 Weight-average (A) 117,560 138,000 107,080 125,800 molecular weight (b-1) 168,000 168,000 168,000 168,000 Specific gravity (A) 0.96 0.96 0.96 0.95 (B) and/or (B)′ 0.96 0.96 0.96 0.96 Specific gravity ratio ((B) and/or (B)′)/(A) 1.00 1.00 1.00 1.01 Evaluations Ball deflection (mm) 3.25 3.25 3.30 3.30 Releasability from mold good good good good Durability of markings good good good good Color uniformity good good good good *The amount of magnesium stearate (component (b-4)) included in the masterbatch is given here as the content per 100 parts by weight of component (A) and component (b-1) combined.

TABLE 5 Comparative Example 1 2 3 Core Material A A A Diameter (mm) 38.55 38.55 38.55 Cover Thickness (mm) 2.07 2.07 2.07 Color clear white yellow Type of masterbatch colorant dye Formulation (A) Himilan 1601 42.5 42.5 (pbw) Himilan 1557 42.5 42.5 Himilan 1605 50 Himilan 1706 50 Nucrel AN4319 15 15 (B) Color masterbatch No. 1 7.5 Color masterbatch No. 2 (B)′ Masterbatch No. 3 7 3.5 Magnesium stearate* 1.86 0.90 Weight-average (A) 125,980 148,000 125,980 molecular weight (b-1) — 61,000 133,955 Specific gravity (A) 0.95 0.96 0.95 (B) and/or (B)′ — 1.64 1.17 Specific gravity ratio ((B) and/or (B)′)/(A) — 1.71 1.23 Evaluations Ball deflection (mm) 3.36 3.14 3.34 Releasability from mold NG good good Durability of markings good NG good Color uniformity — good NG *The amount of magnesium stearate (component (b-4)) included in the masterbatch is given here as the content per 100 parts by weight of component (A) and component (b-1) combined.

Details on the ingredients shown in Tables 3 to 5 are given below.

-   Himilan 1601: A sodium-neutralized ethylene-methacrylic acid     copolymeric ionomer resin available from DuPont-Mitsui Polychemicals     Co., Ltd.; acid content, 10 wt %; Mw, 157,000; MFR, 1.3 g/10 min -   Himilan 1557: A zinc-neutralized ethylene-methacrylic acid     copolymeric ionomer resin available from DuPont-Mitsui Polychemicals     Co., Ltd.; acid content, 12 wt %; Mw, 94,600; MFR, 5.5 g/10 min -   Himilan 1605: A sodium-neutralized ethylene-methacrylic acid     copolymeric ionomer resin available from DuPont-Mitsui Polychemicals     Co., Ltd.; acid content, 15 wt %; Mw, 152,000; MFR, 2.8 g/10 min -   Himilan 1706: A zinc-neutralized ethylene-methacrylic acid     copolymeric ionomer resin available from DuPont-Mitsui Polychemicals     Co., Ltd.; acid content, 15 wt %; Mw, 144,000; MFR, 0.9 g/10 min -   Nucrel AN4319: An ethylene-methacrylic acid-acrylic acid ester     terpolymer available from DuPont-Mitsui Polychemicals Co., Ltd.; Mw,     127,000; MFR, 60 g/10 min

The physical properties of the covers and golf balls obtained in the respective Working Examples of the invention and the Comparative Examples were evaluated based on the following criteria. The results are shown in Table 1.

Ball Deflection:

The deflection (mm) of the ball when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) was measured.

Releasability from Mold:

The ease of removing the golf ball from the mold after molding of the ball outermost layer was rated according to the following criteria.

-   -   Good: Separation from the mold was good, with no sticking of the         molded material to the mold.     -   NG: Separation from the mold was poor, with some sticking of the         molded material to the mold.

Durability of Markings:

The appearance of the markings that had been printed onto the surface of the golf ball was visually examined, in addition to which the following abrasion test was carried out.

Abrasion Test:

Ten golf balls and 3 liters of bunker sand were placed in a magnetic ball mill having an 8 liter capacity and mixing was carried out for 144 hours, following which the balls were visually examined for any loss of the markings and to assess the degree of surface scratching, the degree of loss of luster and the degree of sand adhesion.

The rating criteria were as follows.

-   -   Good: No peeling or loss of markings.     -   Fair: No peeling of markings, but slight loss of markings was         observed in abrasion test.     -   NG: Some peeling of markings; also, significant loss of markings         was observed in abrasion test.

Color Uniformity:

The golf ball was visually examined, and the overall appearance of the ball was rated according to the following criteria.

-   -   Good: Overall cover was uniformly colored.     -   NG: Color was uneven. 

1. A golf ball comprising a core and a cover of at least one layer encasing the core, wherein an outermost layer of the cover is formed of a resin composition comprising: (A) a thermoplastic resin, and (B) a color masterbatch, which color masterbatch (B) includes (b-1) a thermoplastic resin, (b-2) a colorant, (b-3) titanium oxide, (b-4) a fatty acid metal salt, and (b-5) a dispersant, component (b-4) being included in an amount of from 0.3 to 1.4 parts by weight per 100 parts by weight of component (A) and component (b-1) combined, the ratio (B)/(A) of the specific gravity of component (A) to the specific gravity of component (B) being from 0.8 to 1.2, and component (A) and component (b-1) having weight-average molecular weights (Mw) of at least 100,000.
 2. The golf ball of claim 1, wherein the thermoplastic resin (A) is one or more selected from the group consisting of: (a-1) an olefin-unsaturated carboxylic acid copolymer, (a-2) a metal ion neutralization product of an olefin-unsaturated carboxylic acid copolymer, (a-3) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester terpolymer, and (a-4) a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester terpolymer.
 3. The golf ball of claim 2, wherein at least 30 wt % of the unsaturated carboxylic acid included in (a-2) the metal ion neutralization product of an olefin-unsaturated carboxylic acid copolymer and/or (a-4) the metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester terpolymer is neutralized with metal ions.
 4. The golf ball of claim 2, wherein (a-2) the metal ion neutralization product of an olefin-unsaturated carboxylic acid copolymer and/or (a-4) the metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester terpolymer include as the metal ions one or more selected from the group consisting of Li⁺, Na⁺, Mg⁺⁺, Ca⁺⁺ and Zn⁺⁺.
 5. The golf ball of claim 2, wherein component (A) further includes (a-5) one or more selected from the group consisting of polyethylene, thermoplastic polyurethane and rubbery elastomer.
 6. The golf ball of claim 1, wherein the thermoplastic resin serving as component (b-1) is one or more selected from the group consisting of: (b-1-1) an olefin-unsaturated carboxylic acid copolymer, and (b-1-2) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester terpolymer.
 7. The golf ball of claim 1, wherein the thermoplastic resin serving as component (b-1) has a melt flow rate (MFR) of from 5 to 60 g/10 min.
 8. The golf ball of claim 1, wherein the colorant (b-2) is one or more selected from the group consisting of red, blue, yellow, orange, green, violet and black colorants.
 9. The golf ball of claim 1, wherein the fatty acid metal salt (b-4) is a metal salt of a fatty acid having from 16 to 30 carbons.
 10. The golf ball of claim 1, wherein the fatty acid component of the fatty acid metal salt (b-4) is one or more selected from the group consisting of stearic acid, oleic acid, behenic acid and montanic acid.
 11. The golf ball of claim 1, wherein the metal ion of the fatty acid metal salt (b-4) is one or more selected from the group consisting of Li⁺, Na⁺, Mg⁺⁺, Ca⁺⁺ and Zn⁺⁺.
 12. The golf ball of claim 1, wherein component (B) is included in an amount of from 3 to 25 parts by weight per 100 parts by weight of component (A). 